He received about $539,000 of cash contributions from January through March and spent a little less than $130,000. When adding money he previously raised, Koster's balance stood at just over $2 million.

That's significantly higher than his potential challengers.

Republican Catherine Hanaway launched her gubernatorial bid earlier this year and has raised a little over $300,000.

Republican State Auditor Tom Schweich has nearly $835,000 in his campaign account. He's faces only a Libertarian and Constitution party candidate in his re-election bid this year and could carry over some of that money to a potential 2016 governor's race.